Apparatus for exchanging the filament of an electron gun



Sept. 30, 1969 I YQSHIO SAKAMOTO 3, 2

APPARATUS FOR EXCHANGING THE FILAMENT OF AN ELECTRON GUN Original Filed June 12, 1963 3 Sheets-Sheet l p 1 969 YOSHIO sAkAMoTo 3 70,

APPARATUS FOR EXCHANGING THE FILAMENT OF AN ELECTRON GUN Original Filed June 12, 1963 3 Sheets-Sheet 2 P 0, 1969 YOSHIO SAKAMOTO 3,470,412

APPARATUS FOR EXCHANGING THE FILAMENT OF AN ELECTRON GUN S Sheets-Sheet 3 Original Filed June 12, 1963 \NNN NNNNN N United States Patent U.S. U. 313--237 6 Claims ABSTRAiZT 0F THE DKSCLOSURE A device for exchanging the filament of an electron gun without breaking airtightness of the casing enclosing the insulating tower which contains the electron gun as sembly. The exchange of the filament is carried out through the utilization of an elongated sleeve airtightly carried by the top wall of the casing and arranged in coaxial alignment with the insulating tower. The sleeve has a detachable airtight lid at its bottom and is axially movable from an entirely retracted position at the level of the top wall to a plunged position within the easing where the sleeve is engageable airtightly with the insulating tower.

This application is a continuation of application S.N. 287,444, filed June 12, 1963, now abandoned.

This invention relates to an airtight closure for an electron gun which is included in the electron microscope, electron accelerator, or electron diffraction apparatus, and more particularly to an airtight closure for an electron gun in which the filament is exchangeable.

The illuminating system of the electron microscope includes a source of electrons which is called the electron gun. The actual source of electrons in most modern electron microscopes is a pure tungsten wire, the filament, heated to incandescence. This filament is usually bent into the shape of a sharp V. At a short distance immediately in front of the filament is an apertured disk, the Wehnelt cylinder, which serves as a focusing electrode. It is usually either at the same potential or a few volts negative with respect to the filament. A high negative potential, the accelerating potential, is applied to the filament and focusing aperture system which are insulated from the remainder of the instrument. Though the accelerating potential in most commercial microscopes is in the range 30-100 kilovolts, potentials as high as 300 kilovolts or more have been investigated and found useful. The electron gun also includes an anode structure for accelerating the electrons emitted from the filament. It is arranged coaxially with the filament-focusing aperture assembly and at a distance in front of it sufficient for the prevention of electrical breakdown. In case of such an ultrahigh potential as 300 kilovolts or more, the anode structure consists, preferably, of a plurality of anodes having different potentials and being arranged in coaxial alignment. These anodes are shaped either in a tubular form or in an apertured disk and must be also spaced for the prevention of electrical breakdown between each adjoining two. The electron source must be maintained in a high vacuum. Thus, the electron gun assembly is enclosed in a vacuum tube which is, preferably, made of an insu lating material. In case that the anode structure consists of several anodes having different potential, the vacuum tube may be shaped in a tower of several stories. The filament is enclosed in the insulating tower at its top section. In order to prevent electrical leakage along the outside surface of the insulating tower which occurs 3,470,412 Patented Sept. 30, 1969 due to the fact that somewhat conductive impurities are accumulated on the insulating material, the insulating tower is preferably, further enclosed in an airtight casing which is filled with highly insulating gas. When the filament of the electron gun is off, the introduction of the new filament into the insulating tower for exchange presents a difficult problem.

The principal object of the invention is to provide a new and improved airtight closure for an electron gun in which the filament is exchangeable without breaking airtightness of the casing enclosing the insulating tower which in turn contains the electron gun assembly.

The filament-Wehnelt cylinder assembly must be arranged accurate coaxial alignment with the anode structure. Another object of the invention is to provide means for changing the old filament for a new one without putting the coaxial alignment in the anode structure into disorder.

The invention is applied in an airtight closure comprising an airtight casing which encloses an insulating tower containing the electron gun assembly. According to the invention, the insulating tower is airtightly closed at its top Within the airtight casing by a detachable plug which may be the filament support of the electron gun assembly. In addition to this, the airtight casing of the invention is provided at its top wall with an elongated sleeve having a detachable airtight lid thereon. The sleeve is arranged in coaxial alignment with the insulating tower and axially movable from an entirely retracted position at the level of said top Wall to a plunged position within said casing at which said sleeve is engageable airtightly with said insulating tower.

The device constructed as above enables the small section in the casing to be separated from the remainder so that the insulating tower may be communicated through the sleeve directly to the exterior of the casing. At this state the filament in the insulating tower is exchangeable at will.

Other objects and advantages of this invention will be apparent during the course of the following detailed description.

In the accompanying drawings, forming a part of this specification, and wherein similar reference characters designate corresponding parts throughout the several views:

FIG. 1 is a front view of an airtight closure for an electron gun according to the invention, partly in section and partly cut off;

FIG. 2 is a plan view of FIG. 1;

FIG. 3 is a vertical sectional view on an enlarged scale of the sleeve which is particularly provided according to the invention;

FIG. 4 is a sectional view taken along the line 44 in FIG. 3;

FIG. 5 is a vertical sectional view on an enlarged scale of the top portion of the insulating tower illustrated in FIG. 1; and

FIG. 6 is a plan view of FIG. 5.

Referring now in detail to the drawings, the electron gun is enclosed in an insulating tower generally indicated as 11. The electron gun may be of any known type as the electron source in electron microscopes, electron accelerators or electron diffraction apparatus and comprise a filament, a Wehnelt cylinder and an anode structure. The filament is usually bent into the shape of a sharp V and so arranged in the electrodes of the gun that only those electrons emitted by a small area at the tip of the V are utilized in the illumination of the specimen. The Wehnelt cylinder is an apertured disk which is at a short distance immediately in front of the element and serves as a focusing electrode. It is usually either at the same potential or a few volts negative with respect to the filament.

Such a ultra-high negative potential as 500 kilovolts is applied to the filament and focusing aperture system which are insulated from the remainder of the instrument. The anode structure may comprise a plurality of (for example, five) anodes in the form of an apertured disk or tube which are arranged in coaxial alignment with the filament-focusing aperture assembly. The five anodes have different potentials, for example, the first anode 400 kilovolts, the second 300 kilovolts, the third 200 kilovolts, the fourth 100 kilovolts and the last anode at ground potential with respect to the filament of 500 kilovolts. The anode structure is spaced from the filament-focusing aperture assembly sufiicient for the prevention of electrical breakdown. The five anodes are also arranged in a spaced relationship so as to prevent the electrical breakdown between any adjoining two. The electrons emitted from the filament are accelerated by the anode structure and in turn enter into a body tube. In case of electron transmission electron microscopes, the body tube is provided with a condenser, an objective, a projector and a fluorescent screen. However, the invention can be applied not only to the transmission electron microscopes, but also to any other type of electron microscopes, electron accelerators and electron diffraction apparatus.

In the embodiment illustrated in FIG. 1, the electron gun assembly is shown schematically by dotted lines. The numeral 12 denotes the filament to which an ultra-high negative potential is applied and 13 is the Wehnelt cylinder. 14, 15, 16, 17 and 18 are the accelerating electrode anodes having different potentials, for example, 400 kilovolts, 300 kilovolts, 200 kilovolts, 100 kilovolts and ground potential. is the current carrying cable for supplying high negative potentials to the electrodes of the electron gun. The current carrying cable 10 is covered by an insulating material. The insulating tower 11 may be installed in a five-storied form on a base member 19. The accelerating electrodes 14 to 18 are supported by junction members 20 to 23 inserted between each story and the base 19, respectively. The opening of the last anode 18 on the base member communicates to the electron path in the body tube 24 to form a vacuum chamber.

To the top story 25 of the insulating tower 11 is fixed a mounting base 26 for a plug 27. The mounting base 26 has a flat top surface 28 which is provided at its center with an opening 29. The plug 27 is inserted into the opening 29 of the base 26 and detachably secured therein to close airtightly the insulating tower containing the electron gun assembly. The detachable securing of the plug 27 to the member 26 is preferably accomplished by the use of a bayonet joint 30 (similar to that found in the lens barrel of the photographic camera), but it may be accomplished by any other means, for example, by screwsecuring. 31 is a packing which is inserted between the plug 27 and the base 26 for an airtight sealing therebetween.

The plug 27 may have a support 32 for the filament- Wehnelt cylinder assembly (which will be referred hereinafter merely as the filament support) to form an incorporated body. In this manner, the filament and the Wehnelt cylinder can be detachable together with the plug 27 from the insulating tower. The plug 27 is also provided at its top with a recess 33 at which a handle is engageable with the plug 27 for manipulating. The handle has an engaging element complementary to the recess 33.

The insulating tower 11 and the current carrying cable 10 are vertically installed and enclosed in an airtight casing 35. The inside of the casing 35 is preferably filled with highly insulating gas such as Freon CF Cl sulfur hexafluoride SP or nitrogen N, but it may be made vacuous. Freon will find a preferable condition at two atmospheres while hexafluoride at six atmospheres and nitrogen at four atmospheres.

The casing 35 may comprise a cylindrical body 36 having a top cover plate 37 and a bottom cover plate 38. The insulating tower 11 is vertically installed on the base member 19 which is airtightly fixed to the bottom cover plate 38 of the casing. The top cover plate 37 is spaced above from the top of the insulating tower 11.

The top cover plate 37 forming the top wall of the casing 35 is provided with a movable sleeve. The movable sleeve 41 is illustrated more in detail in FIG. 3. The top cover plate 37 has provided at its center an upwardly extending cylindrical attachment 42 in which an elongated sleeve 41 is fitted so that the sleeve 41 is arranged in coaxial alignment with the insulating tower in the casing 35. The cylindrical attachment 42 is airtightly fixed on the cover plate 37 by locking bolts 43 and a packing ring 44. The sleeve 41 is movable in the axial direction up-and-down, with airtightness being maintained between the sleeve 41 and the cylindrical attachment 42. 45 is a packing for airtight seal between the sleeve 41 and the cylindrical attachment 42, and 45 is its gland. The sleeve 41 has a lid 46 at its bottom opening 47. The lid 46 is detachably and airtightly secured to the sleeve 41 by suitable joint means, for example, by bayonet joint means generally indicated as 48 and a packing seal 49. This may also be accomplished by screw-securing with packing means. In this manner, the casing 35 can be airtightly closed by the sleeve 41 having the lid 46. At the top side of the lid 46 is formed a catch element 50 which is shaped in a cylinder having a pair of T-shaped recesses 51. An elongated double- T-shaped rod handle will be engageable at its one T- shaped end with the catch element 50 for manipulating the lid 46 for detachment from the sleeve 41.

The sleeve 41 is axially movable from an entirely retracted position at the level of the top wall of the casing 35 to a plunged position within the casing 35 where the sleeve 41 is engageable airtightly with the insulating tower 11. At the former position, the bottom flange 52 of the sleeve 41 is received by the stepped cavity 53 which is formed round the bottom opening of the cylindrical attachment 42. 54 is a packing for buffering. At the latter position, the end surface 55 of the flange 52 comes in contact with the top surface 28 of the insulating tower. The seal packing 34 on the top surface 28 gives an airtight engagement between the sleeve 42 and the insulating tower 11. On the other hand, the sleeve 41 is provided at its top end with a loosely fitted cap nut 56. When the sleeve 41 is plunged into the inside of the casing 35 until its bottom end surface 55 becomes in contact with the top surface 28 of the insulating tower 11, the cap nut 56 becomes engageable with a screw-threaded portion 57 at the top of the cylindrical member 42. The sleeve 41 can be clamped toward the insulating tower 11 by locking the nut 56 to the member 42 whereby a completely airtight engagement can be obtained and settled between the sleeve 41 and the insulating tower 11.

The sleeve 41 is, preferably, driven by a pair of chain dr1ve means which are arranged symmetrically on the opposite side of the sleeve 41. Each chain drive means may comprise a drive chain-wheel 61 pivotally carried by a bearing supported on the top cover plate 37 of the casing, a chain 62 substantially vertically stretched from the top flange 63 of the sleeve, a chain box 64 which lies on the cover plate 37 and holds the remained length of the chain over the chain wheel 61. Two chain wheels 61 are simultaneously driven by a single handle shaft 65 through the TBSPCtIVfi gearings 66, which may be worm-wheel gearings.

The sleeve 41 may be provided with a passage 71 for s lpplying vacuum to the front of the lid and a microswitch 72 for detecting the moment when the bottom surface of the sleeve 41 becomes in contact with the top surface of the insulating tower 11.

The operation for changing the broken filament for a new one is as follows:

1) Plunging the sleeve 41 into the casing 35 until the bottom end surface 55 of the sleeve 41 comes in contact with the top surface 28 of the insulating tower 11. At this position, the sleeve 41 is secured to the cylindrical attachment 42 by the engagement of the cap nut 56 and the screwed portion 57.

(2) Breaking the vacuum of the inside of the insulating tower 11 into the atmospheric pressure.

(3) Detaching the lid 46 from the sleeve 41 by using a handle. The handle is inserted into the sleeve and engaged with the lid.

(4) Detaching the plug 27 with the filament support 32 from the insulating tower 11 by using a handle. The handle is inserted into the sleeve and engaged with the plug. Introducing the plug with a new filament support into the insulating tower 11 by using the same handle.

(5) Resetting the inside of the insulating tower 11 to a vacuum condition.

(6) Securing the lid 46 to the sleeve 41.

(7) Making a vacuum in the space between the lid 46 and the plug 27 and then introducing highly insulating gas therein.

(8) Retracting the sleeve 41 until the bottom flange 52 engages with the stepped cavity 53 of the cylindrical attachment 42.

It should be understood, of course, that the foregoing disclosure relates to only preferred embodiments of the invention and that numerous modifications or alterations may be made therein without departing from the spirit and the scope of the invention as set forth in the appended claims.

What I claim is:

1. In an airtight closure for an electron gun assembly including an airtight casing, an insulating tower enclosed in said casing with the top of the insulating tower being spaced from the top wall of said casing, said insulating tower containing said electron gun assembly and being airtightly closed at its top within said casing by a detachable plug, means for maintaining said airtightness of said casing while exposing the interior of said insulating tower to the atmosphere for exchanging said electron gun assembly, said means comprising an elongated sleeve having a detachable airtight lid at its bottom, said elongated sleeve including passage means for evacuating the area beneath said lid, said passage means also permitting an insulating gas to be supplied to said area beneath said lid, and a guide member for airtightly supporting said sleeve in coaxial alignment with said insulating tower, said guide member extending outwardly from the top wall of said casing, and means for driving said sleeve along said guide member from an entirely retracted position at the level of said top wall to a plunged position within said casing where said sleeve is engageable airtightly with said insulating tower surrounding said detachable plug, whereby said detachable lid of said sleeve and said detachable plug of said insulating tower may be removed and said electron gun assembly exchanged while maintaining the airtightness of said casing about said insulating tower and thereafter the space between the replaced detachable plug and said lid member evacuated and filled with an insulating gas.

2. An airtight closure for an electron gun assembly as defined in claim 1, wherein the filament support of the electron gun assembly is mounted onto said detachable plug.

3. In an airtight closure for an electron gun assembly including an airtight casing, an insulating tower enclosed in said casing with the top of the insulating tower being spaced from the top wall of said casing, said insulating tower containing said electron gun assembly and being airtightly closed at its top within said casing by a detachable plug, means for maintaining said airtightness of said casing while exposing the interior of said insulating tower to the atmosphere for exchanging said electron gun assembly, said means comprising an elongated sleeve having a detachable airtight lid at its bottom and a guide member for airtightly supporting said sleeve in coaxial alignment with said insulating tower, said guide member extending outwardly from the top wall of said casing, and means for driving said sleeve along said guide member from an entirely retracted position at the level of said top wall to a plunged position within said casing where said sleeve is engageable airtightly with said insulating tower surrounding said detachable plug, whereby said detachable lid pf said sleeve and said detachable plug of said insulating tower may be removed and said electron gun assemblylexchanged while maintaining the airtightness of said casing about said insulating tower.

4. A device for exchanging an article in a high-pressure vessel which comprises a tubular member, said tubular member being adapted to slide through an opening in the wall of said high-pressure vessel so that at the innermost position of saidtubular member the end face thereof engages a fixed supporting member within said vessel upon which said article to be exchanged is mounted, said tubular member being in a gas-tight, sealed relationship within said high-pressure vessel and permitting said supporting member to be exposed to the atmosphere for exchanging said article, said tubular member further having a detachable airtight lid at its bottom, whereby upon the end face of said tubular member engaging said supporting member when moved to its innermost position, said detachable lid may be removed and said article exchanged while maintaining the gas-tightness of said high-pressure vessel about said supporting member.

5. The device as defined in claim 4, wherein said article is an electrical article.

6. The device as defined in claim 4, wherein said tubular member includes passage means for evacuating the area beneath said detachable lid, said passage means also permitting an insulating gas to be supplied to said area beneath said lid, whereby upon said end face of said tubular member engaging said support member, the area beneath said lid may be evacuated and filled with an insulating gas after said article has been exchanged and said lid replaced.

References Cited UNITED STATES PATENTS 2,292,087 8/ 1942 Ramo 313-237 X 2,301,955 11/1942 Krause 313237 X 2,602,899 7/1952 Page 313--237 X 2,995,658 8/1961 Craig. 3,076,893 2/ 1963 Damoth et al. 3,268,759 8/ 1966 Minamikawa et al. 313238 X FOREIGN PATENTS 1,054,185 4/1959 Germany.

907,223 6/ 1945 France. 1,284,007 1/ 1962 France. 1,294,921 4/ 1962 France.

JOHN W. HUCKERT, Primary Examiner ANDREW J. JAMES, Assistant Examiner U.S. Cl. X.R. 313236, 337 

